This invention relates to fabricating optical waveguide gratings.
Dispersion compensation is an attractive technique allowing the upgrade of the existing installed standard fibre network to operation at 1.5 xcexcm where it exhibits a dispersion of xcx9c(about) 17 ps/nm.km which would otherwise prohibit high capacity (eg. 10 Gbit/s) data transmission.
Chirped fibre gratings are currently the most attractive technique for fibre dispersion compensation [1]. This is because they are generally low loss, compact, polarisation insensitive devices which do not tend to suffer from optical non-linearity which is the case with the main competing technology, dispersion compensating fibre.
For present practical applications chirped gratings must exhibit both high dispersion, xcx9c1700 ps/nm, sufficient to compensate the dispersion of around 100 km of standard fibre at a wavelength of 1.55 xcexcm, and a bandwidth of around 5 nm. This implies a need for a chirped grating of length 1 m.
Fibre gratings are generally created by exposing the core of an optical fibre to a periodic UV intensity pattern [2]. This is typically established using either an interferometer or a phase mask [3]. To date, phase masks are the preferred approach owing to the stability of the interference pattern that they produce. The length of the grating can be increased by placing the fibre behind the phase mask and scanning the UV beam along it. Techniques for post chirping a linear grating after fabrication include applying either a strain [1] or temperature gradient [4] to it. However these techniques are limited due to the length of the initial grating (xcx9c10 cm with available phase masks) and the length over which a linear temperature or strain gradient can be applied. Alternatively more complex step chirped phase masks can be employed [5]. However, all of these techniques are currently limited to a grating length of about 10 cm.
In addition to chirping the grating, it is also sometimes desirable to be able to apodise (window) the gratings to reduce multiple reflections within them and to improve the linearity of the time delay characteristics. A powerful technique has been developed which allows chirped and apodised gratings to be written directly in a fibre, referred to as xe2x80x9cthe moving fibre/phase mask scanning beam techniquexe2x80x9d [6]. This technique is based on inducing phase shifts between the phase mask and the fibre as the phase mask and fibre are scanned with the UV beam. Apodisation is achieved by dithering the relative phase between the two at the edges of the grating. Like all the previous techniques the one draw back with this technique is that it is again limited to gratings the length of available phase masks, xcx9c10 cm at present.
This problem has been overcome in one approach by Kashyap et al using several 10 cm step-chirped phase masks [5]. These are scanned in series to obtain a longer grating. The phase xe2x80x9cglitchxe2x80x9d or discontinuity between the sections is subsequently UV xe2x80x9ctrimmedxe2x80x9d to minimise its impact. However this is a time consuming and costly process. In addition the effect of the UV trimming will vary with grating ageing.
A technique for potentially writing longer gratings has been reported by Stubbe et al [7]. In this case a fibre is mounted on an air-bearing stage and continuously moved behind a stationary grating writing interferometer. The position of the fibre is continuously monitored with a linear interferometer. The UV laser is pulsed to write groups of grating lines with period defined by the writing interferometer. A long grating can be written by writing several groups of grating lines in a linearly adjacent series, with controlled phase between the sections. The phase shift between each group of grating lines is controlled via the linear interferometer and a computer which sets the time the laser pulses. A short pulse, xcx9c10 ns, is required such that the position of the writing lines is effectively stationary and accurately controlled with respect to fibre motion. Having said this, however, jitter in the pulse timing and in the linear interferometer position will give detrimental random phase errors in the grating. Chirped gratings can potentially be fabricated by continuously introducing phase shifts between adjacent groups along the grating. Obviously the maximum translation speed is limited by the number of grating lines written with one laser pulse and the maximum repetition rate of the pulsed laser. It is also proposed in this paper that apodisation is achieved by multiple writing scans of the grating.
This invention provides a method of fabricating an optical waveguide grating having a plurality of grating lines of refractive index variation, the method comprising the steps of:
(i) repeatedly exposing a spatially periodic writing light pattern onto a photosensitive optical waveguide; and
(ii) moving the writing light pattern and/or the waveguide between successive exposures or groups of exposures of the writing light pattern, characterised in that
the successive exposures or groups of exposures overlap so that each of at least a majority of the grating lines is generated by at least two exposures to different respective regions of the writing light pattern.
Embodiments of the invention provide a number of advantages over previous techniques:
1. The realisation that the laser does not have to be pulsed but just has to be on for a particular duty cyclexe2x80x94preferably less than 50% of the period. This allows an externally modulated CW (continuous wave) laser to be used.
2. With this technique the grating lines are re-written by several successive exposures of the writing light beam at every grating period (or integral number of grating periods). Thus the footprint defined by the writing light beam is significantly overlapped with the previous lines. Significant averaging of the writing process is achieved thus improving the effective accuracy and resolution of the system, compared to that of [7] where a group of lines is written in a single exposure, and the fibre is then advanced to a fresh portion where a further group of lines is written in a single exposure.
3. Effectively controlling the grating writing process on a line-by-line basis allows accurate apodisation to be achieved. This may be performed in embodiments of the invention by dithering the grating writing interferometer position in the fibre to wash out or attenuate the grating strength whilst keeping the average index change constant.
4. The technique offers the further advantage that the CW laser may be extremely stable, whereas pulsed lasers (e.g. those used in [7]) may suffer from pulse-to-pulse instability which is not averaged. In addition the high peak powers of the pulsed laser may cause non-linear grating writing effects.
5. Arbitrary phase profiles and in particular a linear chirp can be built up by inducing phase shifts electronically along the grating as it grows. In a similar manner to the xe2x80x9cMoving fibre/phase maskxe2x80x9d technique [6] the maximum wavelength is inversely proportional to the beam diameter. This can be further improved in particular embodiments of the invention by incorporating a short, linearly chirped phase mask. Thus as the fibre is scanned the UV beam may be also slowly scanned across the phase mask, an additional small phase shift is induced, whilst most significantly we have access to writing lines of a different period allowing larger chirps to be built up.
This invention also provides an optical waveguide grating fabricated by a method according to the above methods.
This invention also provides apparatus for fabricating an optical fibre grating having a plurality of grating lines of refractive index variation, the apparatus comprising:
a writing light beam source for repeatedly exposing a spatially periodic writing light pattern onto a photosensitive optical waveguide; and
means for moving the writing light pattern and/or the waveguide between successive exposures or a groups of exposures of the writing light pattern, characterised in that
the successive exposures or groups of exposures overlap so that each of at least a majority of the grating lines is generated by at least two exposures to different respective regions of the writing light pattern.
The various sub-features defined here are equally applicable to each aspect of the present invention.